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“This new software will compliment the existing system.”

Can you spot what’s wrong with that sentence?

If you get confused by the difference between compliment and complement, or if you’re unsure which to use when, read on.

Compliment

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Merriam-Webster defines a compliment as “an expression of esteem, respect, affection, or admiration; especially : an admiring remark”. It comes from Middle French, via the Italian complimento, and the Spanish cumplimiento, which originates from the Latin verb cumplir: to be courteous. For example:

• I was trying to pay that girl a compliment, but she ignored me.
• Sometimes he blushes when you offer him a compliment.

In the plural, compliments can also mean best wishes. It is often used as “with compliments” such as on a compliments slip (a small piece of letter-headed paper, often used by companies for a quick note to a customer or client when a full sheet would be too large.) You also see the phrase “with compliments of the season” in greetings cards. The verb “to compliment” is very similar, meaning “to pay a compliment to”. Note that it is a transitive verb so must have an object. For example:

• Are you trying to compliment me, or trying to insult me?
• When he complimented the girl on her dress, his friends laughed at him.

Complimentary

The adjective complimentary is closely related to the word compliment, and in this context it can mean either “expressing or containing a compliment” or “favourable” (Merriam-Webster):

• My mother made some very complimentary remarks about my choice of shoes.
• The new restaurant has a very complimentary write-up in the local newspaper.

Complimentary also has the meaning “free”, when something is given as a courtesy or favour:

• Please accept these complimentary tickets.
• I thought that the mini-bar was complimentary, but we were charged for our drinks.

Complement

The word complement comes from the same root as complete. It has nothing to do with being courteous, and comes directly from Middle English, from the Latin word complementum. Merriam-Webster’s first definition is “something that fills up, completes, or makes perfect”, and it can also be used to mean “the quantity, number, or assortment required to make a thing complete”, though can sound a little odd or old-fashioned in this context:

• We had the full complement of pots and pans.
• Our store does not have enough employees to work the required complement of hours.

Complement is often used in scientific, technical or academic areas of discourse, where the complement of X supplies what X is missing, thus making a complete whole. Examples of this usage are:

• Complement good (economics)
• Complementary colour (art)

You can find a fuller list in Wikipedia’s entry for the term Complement. In everyday writing, complement is more often used as a verb. Again, it is a transitive verb

:

• The illustrations complement the text.
• Our new software will complement the existing product.

So, the above sentence should have been written, “This new software will complement the existing system.”